/Engine/Plugins/Runtime/MassEntity/Source/MassEntity/Public/MassExternalSubsystemTraits.h
This is a getter function that given a UWorld* fetches an instance.
namespace FMassExternalSubsystemTraits { template<typename T, typename> T * FMassExternalSubsystemTraits::GetInstance ( const [UWorld](API\Runtime\Engine\Engine\UWorld) * World ) }